You don't need to override onBackPressed() - it's already defined as the action that your activity will do by default when the user pressed the back button. So just call onBackPressed() whenever you want to "programatically press" the back button.
That would only result to finish() being called, though ;)
I think you're confused with what the back button does. By default, it's just a call to finish(), so it just exits the current activity. If you have something behind that activity, that screen will show.
What you can do is when launching your activity from the Login, add a CLEAR_TOP flag so the login activity won't be there when you exit yours.